The invention relates to a method according to the preamble of the first independent patent claim. The method serves for determining the spatial orientation of a target tracking mirror in a laser tracking system. The system further relates to a mirror arrangement according to the corresponding independent patent claim, said mirror arrangement serving for carrying out the method.
The target tracking mirror is an essential component of laser tracking systems as they have been known for many years now in industry and are used for precise coordinate measurements on large subjects. Laser tracking systems permit the tracking of a moving target retroreflector with a tracking measurement beam, wherein by way of suitable measurements of the direction of the measurement beam and by way of interferometric measurements of the distance to the target retroreflector the coordinates (e.g. polar coordinates) of the target retroreflector are determined.
Directing the tracking measurement beam in a manner such that it always impinges the moving retroreflector is effected by suitably adjusting the target tracking mirror which usually is rotatable about two axes standing perpendicular to one another. The tracking measurement beam is reflected back by the retroreflector to the target tracking mirror and from this is deflected to the interferometer receiver. The interferometer receiver determines the distance of the target retroreflector to a defined zero position of the laser interferometer. The spatial orientation of the target tracking mirror is determined by measurement and from the readings the direction of the measurement beam is computed.
For rotating the target tracking mirror about the axes usually servomotors assembled on the axes are provided. For determining the orientation of the target tracking mirror, with respect to a pregiven zero orientation usually each of the axes is equipped with an angle encoder. A typical laser tracking system equipped in such a manner with a target tracking mirror is for example described in the publication U.S. Pat. No. 4,714,339. This system comprises a target tracking mirror arranged in a cardanic suspension and therefore being rotatable about two axes standing perpendicular to one another.
In Applied Optics, Volume 2, No. 7, July 1963, page 762 ff. the use of a Michelson interferometer is described as an alternative to angle encoders for angle measurement in order to determine the rotational movement in a gamma beam spectrometer. The Michelson interferometer is a two-armed interferometer with two equally long arms, one for the reference beam path and the other for the measurement beam path. In the described application the measurement beam is directed in an unchangeable manner towards a die-corner prism, the prism being arranged on a part which is rotatable about an axis. The measurement beam is deflected by the prism to a stationary mirror and by the mirror to the same path back to the interferometer. Between the interferometer and prism, the measurement beam runs parallel to a tangent on the circular arc described by the prism on rotating about the axis. The reference beam is reflected by a stationary mirror to the interferometer. With rotation of the prism about the rotational axis the measurement beam path is lengthened or shortened and this is interferometrically detected. From the change of the path length of the measurement beam path when moving the prism from a predetermined zero position to a momentary position the rotational angle of the prism with respect to this zero position is calculated.
The described arrangement of prism and mirror renders it necessary for the measurement beam to eccentrically impinge the same prism side in every possible rotational position of the prism. This means that the opening of the prism and thus the prism itself must be relatively large. Therefore, the prism is relatively heavy so that it may influences the moment of inertia of the rotating part in a relevant manner.
It is the object of the invention to provide a method for determining the spatial orientation of a target tracking mirror in a laser tracking system, wherein the method according to the invention with respect to known methods for determining the spatial orientation of such a target tracking mirror is to permit a higher accuracy and wherein the method is to permit a very compact mirror arrangement in which the parts to be moved with the mirror are to be as light as possible so that as small as possible inertia stands in the way of a movement of the mirror. It is furthermore the object of the invention to provide a mirror arrangement for carrying out the method according to the invention. This arrangement is to be compact and possibly able to be integrated into known laser tracking systems in a modular manner.
This object is achieved by the method and the device as defined in the independent patent claims. Further advantageous embodiments of the invention are the subject-matter of the dependent claims.
The method according to the invention is based on the idea of determining the spatial orientation of the target tracking mirror of the laser tracking system not with the help of angle encoders arranged on rotational axes but with interferometric methods. For this purpose at least two retroreflectors are connected to the target tracking mirror and are therefore moving together with the target tracking mirror. For each one of the reflectors a secondary interferometric measurement system is provided, wherein the measurement and reference beams of the secondary measurement systems are branched out of the primary beam path of the laser tracking system. In each secondary measurement system a secondary measurement beam with an unchangeable direction is directed onto one of the retroreflectors moving with the target tracking mirror and the change of the length of the beam path of the secondary measurement beam is interferometrically determined when the mirror is moved. Retroreflectors and secondary measurement systems are arranged such that mirror movements effect path length changes for the secondary measurement beams and that the measurement data gained from such path length changes which characterize distances of a momentary reflector position from a predetermined zero position in the direction of the measurement beam, allow unambiguous computation of the mirror orientation.
The movements of a target tracking mirror are usually rotation movements in which the retroreflectors allocated to the secondary measurement systems move on a circular arc also. Each secondary measurement beam is directed tangentially onto such a circular arc, advantageously in a manner such that it impinges the retroreflector essentially centrally when located in a middle position. Since the direction of the secondary measurement beam always remains the same but the reflector does not move in a straight line in the measurement beam path but on a circular path, it is evident that the movement of the reflector detected by the measurement beam is limited. The parallel shift of the reflected measurement beam relative to the incident measurement beam which is caused by eccentric incidence of the measurement beam onto the reflector is compensated by beam widening optics.
In order to compute the orientation of the mirror surface of any moving mirror it is theoretically necessary to measure the spatial positions of three points arranged stationary relative to the mirror surface. For determining the orientation of a mirror rotatable about two stationary intersecting axes the evaluation of the position of two such points is sufficient.
The method according to the invention is advantageously carried out using an interferometer with a non-shifted return beam (single beam interferometer) which works according to the heterodyne principle. A frequency difference between the measurement beam and the reference beam is evaluated and therefore an arm for the reference beam path as necessary in the Michelson interferometer is not needed. A part of the laser beam is branched off as a reference beam and is led directly to the interfermometer receiver, the remaining light of the laser beam is frequency-shifted by way of an acousto-optical modulator and serves as measurement beam, i.e. runs through the measurement beam path to the measurement object, is reflected on this and is then led to the interferometer receiver. The interferometer detects the interference, i.e. a signal at a frequency which arises with the superposition of the measurement beam with the reference beam and which corresponds to the modulation frequency plus/minus the Doppler frequency. Subsequent electronics compares this signal with respect to phase and frequency to the original modulation frequency and produces per traversed half wavelength, according to the direction, a positive or negative count impulse.
The secondary measurement beams are advantageously branched out of the beam path of the primary measurement system, after the mentioned modulator so that only one laser source and only one modulator need be provided.
According to the invention, the arrangement of the target tracking mirror for a laser tracking system comprises means for changing the orientation of the mirror from a predetermined zero orientation and at least two retroreflectors co-moving with the mirror. Furthermore, it comprises means for deflecting measurement and reference beams of at least two secondary measurement systems out of the beam path of the laser tracking system, means for directing in each case one secondary measurement beam onto one of the retroreflectors connected to the target tracking mirror, means for interferometric analysis of the secondary measurement beams reflected by the retroreflectors for detecting path length changes in the beam paths as well as means for computing the spatial orientation of the target tracking mirror from the measured path length changes.
The target tracking mirror is for example in a known manner mounted in a cardanic suspension and is rotatable about two axes orthoganal to one another, for which suitable drives are provided.